Among electronic appliances that communicate wirelessly, electronic appliances which operate without wired power supply, such as a portable transceiver, a mobile phone, and a radio frequency identification system (RFID) tag have been particularly required to have as low power consumption as possible.
Among electronic appliances which operate without wired power supply, a potable transceiver and a mobile phone whose power sources are each a battery incorporated therein have been required to have as low power consumption as possible in order to increase time in which they can be used per charge.
In electronic appliances which operate without wired power supply, there are also an active RFID tag and a passive RFID tag. Since a passive RFID tag which does not incorporate a battery operates by electromagnetic waves from an interrogator, as power, the passive RFID tag is required to operate even when only a slight amount of power is generated in the case where a distance from the interrogator is increased and thus only weak electromagnetic waves can be received. Therefore, the passive RFID tag is particularly required to have low power consumption.
Here, an example is briefly described in which a passive RFID tag communicates wirelessly.
In the case where a passive RFID tag conducts reception through wireless communication, the passive RFID tag receives carrier waves which are electromagnetic waves transmitted from an interrogator by an antenna, converts the carrier waves into an AC signal, and rectifies the AC signal in a rectifier circuit, so that a DC voltage to be power is generated.
On the other hand, since data to be received by a passive RFID tag is transmitted through carrier waves modulated by a signal obtained by encoding the data in the interrogator, the passive RFID tag obtains data in such a manner that carrier waves are received by an antenna and the received carrier waves are demodulated into an encoded signal in a demodulation circuit. Further, in the case where the passive RFID tag is mounted with a clock synchronization-type logic circuit, in order to perform a process in accordance with the received data, the passive RFID tag generates a clock by dividing a frequency of an AC signal converted by the antenna into a desired frequency.
Further, the passive RFID tag makes each internal circuit operate with the generated voltage, performs a process in accordance with data received in synchronization with the generated clock, and responds to the interrogator if necessary.
Incidentally, when the passive RFID tag performs a process in accordance with received data, it decodes the encoded signal obtained by demodulation.
Among various ways for encoding, pulse interval encoding (PIE) is taken as an example here. By PIE, data of logical values 0 and 1 is encoded into signals with pulses generated at different time intervals. Examples of PIE are described with reference to FIGS. 2 and 3.
In FIG. 2, a signal 200 with a pulse generated at a short time interval is indicated as data of a logical value 0, and a signal 201 with a pulse generated at a long time interval is indicated as data of a logical value 1. FIG. 3 illustrates a signal 500 in the case where consecutive data, for example, 01011 is encoded.
The passive RFID tag determines the interval of time from the rise of a pulse to the next rise thereof by calculating the number of oscillation of a generated clock, in order to take data from the signal obtained by encoding by PIE, decodes the encoded signal into data of logical values 0 and 1 in accordance with the obtained number of oscillation, stores the encoded data in a storage element such as a register, and performs a process in accordance with the data.
As such a passive RFID tag which communicates wirelessly, for example, Non-Patent Document 1 is given.